This invention relates to electrostatic printing and, more particularly, to a method and apparatus for applying a charge pattern in an image configuration to a charge-retentive imaging member by the utilization of an electrode array.
Electrostatic printing in its simplest form consists of producing charge patterns on a suitable surface by application of an electric field and then rendering the charge patterns visible and fixed by use of a suitable developer material. Some prior art electrostatic printing devices use a recording head, having a plurality of electrodes in what is known as a stylus array, and a recording medium typically in the form of a dielectric coated paper or web. While the recording medium is urged into engagement with the styli electrodes, a sufficient voltage is developed across selected styli and the recording medium to cause selective charging of the recording medium. The electrostatic pattern is then made visible in a suitable developer station by suitable developer materials.
In certain approaches, the stylus array maintains a gap between the electrodes and the recording medium so as to charge an electroreceptor in accordance with digital image information fed to the array. In this regard, the electrodes preferably make no contact with the recording medium; the voltage applied to the electrodes causes electrical breakdown of the air between the styli and the insulating electroreceptor, thereby charging the electroreceptor in accordance with the information applied to the array. Those familiar with electrical gas discharges will appreciate that the applied voltage necessary to initiate air breakdown and generate a plasma-discharge between styli tips and the electroreceptive surface is at a minimum at a stylus spacing of a few microns rather than at closer proximity, as described by a Paschen relationship. Consequently, the electrodes, or portions thereof, must be maintained at a proximate but non-contacting distance from the recording medium in order that a finite air gap be present between the recording medium and the electrodes, thereby permitting a conductive plasma to be generated with minimum applied voltage. Subsequently, the recording medium can be toned in the usual fashion followed by transfer to a suitable substrate, such as paper or the like, in the customary fashion. However, in such non-contact systems, the image resolution suffers because of the spreading of the plasma discharge in contact with the recording medium, and the difficulty in controlling the amount of charge deposited by the electrodes at a given applied voltage. The voltage drop in the plasma depends critically on the electrode spacing, so such systems require close tolerances with regard to the gap between the electrodes and the electroreceptor in order to maintain a constant level of deposited charge on the electroreceptor. Further, since there is an unavoidable voltage drop due to the dynamics of the plasma through which the charging current flows, the resulting potential of an electroreceptor surface charged by this technique is substantially less than the voltage applied to the electrodes.
Other approaches to imagewise charging of a recording medium by a recording head involve direct contact of the electrodes to the recording medium. Some such systems utilize a multi-stylus recording head that is transported in an oscillatory fashion across the recording medium while the electrodes are intended to maintain contact with the surface of the recording medium. In this approach, each electrode engages the recording medium typically under a significant pressure, which causes an undesirable tribocharging of the recording material (even in the absence of an applied voltage to the electrode). In order to counteract the effects of the triboelectric charging of the recording material, a compensating direct current voltage may be applied to the electrode. Also, a fairly high contact pressure is required between each electrode and the recording material to ensure that all the electrodes charge the substrate in a consistent and simultaneous manner. As a result, there is abrasion of the styli and the recording medium and disadvantageous deformation of the recording medium due to the contact pressure of the styli against the recording medium.
Accordingly, in one embodiment of the present invention, there is an improved method and apparatus for selectively charging an electrostatic recording medium, wherein a multiple electrode array recording head includes an array of electrodes having recording electrode tips disposed in a flexible insulating layer. The recording electrode tips are presented in a recording head tip that is sufficiently flexible such that the recording head tip may be aligned against an image recording surface of the electrostatic recording medium, whereupon the distal edge of the insulating layer conforms to the image recording surface and the recording electrode tips are placed in direct contact with the electrostatic recording medium. Relative movement between the recording head and the electrostatic recording medium may be provided in a conventional manner such that an electrostatic latent image may be deposited in the electrostatic recording medium.
In another embodiment of the present invention, there is an improved electrostatic recording medium wherein a charge retentive layer is located on a conductive substrate. An image recording surface on the charge retentive layer includes an array of discrete, conductive segments present in the image recording surface. The improved electrostatic recording medium is useful in contact electrostatic printing, wherein an array of recording electrodes may be operated in direct contact with the conductive segment array, with less surface degradation and without resort to a high contact pressure for the traditional efforts to counteract the effects of triboelectric charging of the recording material.
In another embodiment of the present invention, the multiple electrode array recording head and the electrostatic recording medium may be operated in an improved electrostatic printing apparatus. The electrostatic recording medium is constructed to include a charge retentive layer located on a conductive substrate. An image recording surface on the charge retentive layer includes an array of discrete, conductive segments present in the image recording surface. The electrostatic recording head includes a multiple electrode assembly having recording electrode tips that are aligned for engagement with the image recording surface. Relative movement between the recording head and the electrostatic recording medium may be provided in such a manner that certain members of the recording electrode tips momentarily achieve Ohmic contact with respective elements of the discrete conductive segments present in the image recording surface. The charge retentive layer receives an imagewise pattern of localized, discrete charges according to a varying charging voltage present at the tips of the recording electrodes during the respective moments of Ohmic contact. The Ohmic nature of the contact provides charging of the discrete conductive segments to a potential substantially equal to the voltage of the recording electrode. The resulting electrostatic latent image may then be made visible by operation of a developer station by use of suitable developer materials.